This invention relates to data transmission systems and more particularly to digital data transfer throughout a network of subscribers.
It has become increasingly important to be able to transfer digital data from point-to-point at high data rates and at low cost throughout a large network of subscribers. Available communications channels have a finite bandwidth and thus present theoretical limits to the rate that information can be transferred. As a practical matter, it is important to achieve as efficient a data transfer as possible within the bandwidth available.
Data in digital format is presently transmitted point-to-point by a variety of systems and methods. Telephone line systems employ a MODEM at each end of a telephone line. A transmitting subscriber uses a MODEM at one end of the telephone line to encode the data into audio tones for transmission to the MODEM at the other end of the telephone line where the data is decoded. Typically, a 9600 bits/second data rate can be achieved over a telephone data link.
However, it has heretofore not been possible to achieve data rates approaching the telephone line data rate in data transfer systems utilizing the subsidiary communications channels available on commercial broadcast FM radio stations.
Conventional digital data transfer via a radio sub-carrier utilize a frequency shift keying (FSK) system. Digital data is coupled into an FSK MODEM. The FSK MODEM frequency modulates an audio carrier (approximately 1800 Hz.) The audio carrier is shifted between two different frequencies, one corresponding to a "1" and the other corresponding to a "0". This frequency shift keyed (FSK) audio signal is then used to frequency modulate a sub-carrier with a center frequency of 67 KHz. The modulated sub-carrier is then added to the commercial program stereo signal, and the composite signal is used to modulate the FM carrier. This conventional data transfer technique allows the use of off the shelf FSK MODEM equipment and no modification of the broadcast FM radio station equipment is required. In essence, an audio channel is being provided and data is transmitted as audio tones. The rate of digital data transfer for such systems is limited. The data rate limitation for conventional systems using frequency modulation arises because their baud rate cannot be increased without violating bandwidth and attenuation requirements imposed by FCC regulation.
FCC regulations require that the amplitude of the sub-carrier of an FM broadcast radio station be 20 dB attenuated from the level created by full modulation and that harmonics from the modulation of the sub-carrier be 60 dB attenuated for all frequencies below 53 KHz (see FCC regulations .sctn.73.319 Subsidiary communications multiplex operations: engineering standards). Heretofore, in order to achieve high data rates without exceeding FCC guidelines, it would have been necessary to utilize complicated phase modulation (PM) techniques. Such techniques require the use of costly equipment at both ends of the data link.
In order to overcome the rate of data transfer limitation associated with transmitting digital data via the conventional FSK modulation scheme over a commercial broadcast FM radio station, and to provide a more economical system and data communications network, a data transmission system is provided which utilizes a linear modulation technique allowing significantly higher rates of data transfer, typically in the range of 4800 bits/second while maintaining a low error rate and while meeting all FCC bandwidth and attenuation standards.